In a typical forward converter, a main switch is placed on the primary side of the transformer and connected in series with the primary winding of the transformer, and a rectifier circuit which is made up of rectifying diodes is placed on the secondary side of the transformer. The magnetizing inductance of the primary winding of the transformer is configured to receive the current from the voltage input terminal of the forward converter and store energy therein, and transfer the stored energy to the secondary side of the transformer according to the on/off operations of the main switch. The rectifier circuit disposed at the secondary side of the transformer is used to rectify the AC voltage induced on the secondary side of the transformer into a rectified DC voltage. The rectified DC voltage is then smoothed to generate an output DC voltage for use by a load. Because diodes can cause considerable conduction loss during the switching operation, the rectifying diodes within the rectifier circuit have been replaced with transistors so as to implement a synchronous rectifier in a conventional forward converter. The forward converter using a synchronous rectifier can reduce the power loss of the converter and improve the overall efficiency of the converter. However, transistor is a circuit element with bidirectional conductivity, and thus requires a sophisticated driving circuit to drive the synchronous rectifier switch of the synchronous rectifier.
FIG. 1 shows the circuit construction of a conventional synchronous rectifier forward converter. The synchronous rectifier forward converter of FIG. 1 includes a transformer T1 having a primary winding Np and a secondary winding Ns. One end of the primary winding Np is connected to an input DC voltage Vin and configured to store the energy from the input DC voltage Vin in the magnetizing inductance of the primary winding (not shown). The other end of the primary winding Np is connected in series with a main switch Q1 whose switch operation is manipulated by a pulse-width modulator (PWM) 50. The main switch Q1 is generally implemented by a MOSFET having a drain terminal connected to the primary winding Np and a gate terminal connected to the pulse-width modulator 50 and a source terminal connected to ground. The energy stored in the primary side of the transformer T1 is transferred to the secondary side of the transformer T1 according to the on/off operations of the main switch Q1, and thereby inducing an AC voltage across the secondary winding Ns. A synchronous rectifier (Q2, Q3) and a filtering circuit (Lo, Co) are placed on the secondary side of the transformer T1, in which the synchronous rectifier (Q2, Q3) is configured to perform switching operation in synchronization with the main switch Q1 so as to convert the AC voltage across the secondary winding Ns into a rectified DC voltage. The rectified DC voltage is processed by the filtering circuit which is made up of an output inductor Lo and an output capacitor so that the high-frequency harmonics of the rectified DC voltage is removed, in which the output inductor Lo is implemented in the form of a transformer. Therefore, an output voltage Vout is generated across the output capacitor Co and provided to a load Ro.
The synchronous rectifier shown in FIG. 1 is a self-driven synchronous rectifier including a forward switch Q2 and a freewheel switch Q3. The forward switch Q2 also has a drain terminal connected to the secondary winding Ns, a gate terminal connected to one end of an auxiliary winding Na located at the secondary side of the transformer T1, and a source terminal connected to the negative voltage output terminal of the forward converter. The secondary voltage induced across the auxiliary winding Na is used as a gate driving signal for the forward switch Q2. The freewheel switch Q3 also has a drain terminal connected to the positive voltage rail of the output DC voltage Vout, a gate terminal connected to the auxiliary winding of the output inductor Lo, and a source terminal connected to negative voltage output terminal of the forward converter. The voltage induced across the auxiliary winding of the output inductor Lo is used as a gate driving signal for the freewheel switch Q3. The operation of the synchronous rectifier of FIG. 1 is described as follows. When the main switch Q1 is ON, the energy stored in the primary side of the transformer T1 is transferred to the secondary side of the transformer T1, and thereby inducing a positive voltage across the secondary winding Ns. In the meantime, the gate driving signal received by the gate terminal of the forward switch Q2 is a positive voltage and thus the forward switch Q2 is turned on. Hence, the forward switch Q2 can provide a current path between the secondary winding Ns and the negative voltage output terminal of the forward converter, such that an inductor current IL can flow from the secondary winding Ns to the output inductor Lo so as to charge the output inductor Lo, and thereby generating a positive voltage across the main winding of the output inductor Lo. Because the polarity of the main winding of the output inductor Lo is reverse to the polarity of the auxiliary winding of the output inductor Lo, a negative voltage is induced across the auxiliary winding of the output inductor Lo. Therefore, the gate driving signal received by the gate terminal of the freewheel switch Q3 is a negative voltage, and thereby turning off the freewheel switch Q3. When the main switch Q1 is OFF, the transformer enters the reset process, and thereby inducing a negative voltage across the secondary winding Ns. In the meantime, the gate driving signal received by the gate terminal of the forward switch Q2 is a negative and thereby turning off the forward switch Q2. Here, the energy stored in the output inductor Lo is discharged to the output capacitor Co by the inductor current IL, and thereby generating an output DC voltage Vout across the output capacitor Co and inducing a negative voltage across the main winding of the output inductor Lo. Because the polarity of the main winding of the output inductor Lo is reverse to the polarity of the auxiliary winding of the output inductor Lo, a positive voltage is induced across the auxiliary winding of the output inductor Lo. Here, the gate driving signal received by the gate terminal of the freewheel switch Q3 is a positive voltage, and thereby turning on the freewheel switch Q3. Hence, the freewheel switch Q3 can provide a current path between the positive voltage rail and the negative voltage output terminal of the forward converter.
Although the synchronous rectifier can provide several advantages such as low power loss and high conversion efficiency, some potential risks would occur at the instant of the start-up or the shutdown of the forward converter. The major risk is caused by the reverse current which flows from the output capacitor Co to the secondary winding Ns. As stated above, the synchronous switch (Q2, Q3) is made up of transistors having bidirectional conductivity. Therefore, the synchronous switch (Q2, Q3) requires a driving circuit to control its on/off operations. However, no matter whether the synchronous switches of the synchronous rectifier (Q2, Q3) uses a self-driven mechanism or a control-driven mechanism, the source of the gate driving signal stems from the pulse-width modulator 50. Therefore, when the forward converter shuts down or the input power of the forward converter is interrupted, the pulse-width modulator 50 will cease operation and thus the gate driving signal for manipulating the synchronous rectifier (Q2, Q3) will be stopped as well, and the forward switch Q2 will turn off accordingly. Nonetheless, the gate terminal of the freewheel switch Q3 still keeps the residue energy remained during the ON period. Such situation is particularly feasible under a light-load or a no-load condition. Hence, a current loop is formed between the output inductor Lo and the output capacitor Co due to the ever-conducting freewheel switch Q3. Here, the voltage across the secondary winding Ns is zero. Accordingly, the flow direction of the inductor current is IL reversed. Because the gate terminal of the freewheel switch Q3 is driven in a continuous manner, the amount of the reverse current will increase as well until the energy of the gate terminal of the freewheel switch Q3 has dropped down to be smaller than the threshold voltage. Under this condition, the freewheel switch Q3 will turn off and the variance of the reverse current causes sharp voltage spikes between the drain terminal and the source terminal of the synchronous switches Q2 and Q3. These voltage spikes would damage the power semiconductor devices within the synchronous rectifier. What is worse, the instantaneous voltage value of these voltage spikes would exceed the rated voltage of the power semiconductor devices and burn down the power semiconductor devices.
FIGS. 2(A) to 2(E) illustrate various operating modes of the synchronous rectifier forward converter of FIG. 1 during the shutdown process, and FIG. 3 illustrates the waveforms of the inductor current, the gate-source voltage and the drain-source voltage of the freewheel switch within the synchronous rectifier forward converter of FIG. 1. FIG. 2(A) illustrates the operating mode of the synchronous rectifier forward converter during a normal operating process, in which the pulse-width modulator 50 is configured to continuously provide pulse-width modulation signals to the gate of the main switch Q1. Therefore, the energy stored in the primary side of the transformer T1 can be transferred to the secondary side of the transformer T1. In the meantime, the flow of the inductor current IL is directed from the secondary winding Ns to the output capacitor Co. When the main switch Q1 is ON, the forward switch Q2 is turned on to enable the output inductor Lo, the output capacitor Co and the forward switch Q2 to form a current loop. When the main switch Q1 is OFF, the freewheel switch Q3 is turned on to enable the output inductor Lo, the output capacitor Co and the freewheel switch Q3 to form a current loop. The waveform diagram for illustrating this operation mode is represented in the time period t0 to t1 of FIG. 3.
FIG. 2(B) illustrates the operating mode of the synchronous rectifier forward converter as the forward converter is shut down or the input power of the forward converter is interrupted. Under this condition, the power supply for the pulse-width modulator 50 is unavailable, and thus the pulse-width modulator 50 can not provide pulse-width-modulation signals so that the energy transfer initiated by the primary side of the transformer T1 is stopped. Therefore, the voltage across the secondary winding Ns is zero and the secondary winding Ns can not charge the output inductor Lo to store energy in the output inductor Lo. In the meantime, the gate voltage of the freewheel switch Q3 is not dropped down to zero immediately. That is, the freewheel switch Q3 can not be turned off immediately but maintain conducting for a short time, which causes the voltage across the output capacitor Co to be larger than the voltage across the secondary winding Ns. Therefore, the output inductor Lo, the output capacitor Co and the freewheel switch Q3 form a current loop, in which the inductor IL reversely flows from the drain terminal of the freewheel switch Q3 to the source terminal of the freewheel switch Q3 so that the output capacitor Co can charge the output inductor Lo. The waveform for illustrating this operating mode is represented in the time period of t1 to t2 of FIG. 3.
FIG. 2(C) illustrates the operating mode subsequent to the operating mode of FIG. 2(B). In this mode, the output capacitor Co will continuously charge the output inductor Lo. Because the gate voltage of the freewheel switch Q3 has not decayed to the threshold voltage, the reverse current from the output capacitor Co will continue flowing. The waveform for illustrating this operating mode is represented in the time period of t2 to t3 of FIG. 3.
FIG. 2(D) illustrates the operating mode subsequent to the operating mode of FIG. 2(C). In this mode, the gate voltage of the freewheel switch Q3 will be decayed to be smaller than the threshold voltage so as to turn off the freewheel switch Q3. In the meantime, the reverse current flowing through the output inductor Lo will be maximized. Because the initial value of the junction capacitance between the source and the drain of the forward switch Q2 and the freewheel switch Q3 is zero, the instantaneous current of the reverse current will charge the junction capacitance. Therefore, voltage spikes will occur between the drain and source of the forward switch Q2 and the freewheel switch Q3. The waveform for illustrating this operating mode is represented in the time period of t3 to t4 of FIG. 3.
FIG. 2(E) illustrates the operating mode subsequent to the operating mode of FIG. 2(D). In this mode, when the reverse current is maximized, the cycle of the resonance between the output inductor Lo and the output capacitor Co will be finished. In the meantime, the reverse current will diminish and the flow of the inductor current IL will revert to the operating mode of FIG. 2(A). Here, the freewheel switch Q3 will turn on again by the gate driving signal provided by the auxiliary winding of the output inductor Lo, and the inductor-capacitor resonance of the next cycle will start. The waveform for illustrating this operating mode is represented in the time period of t4 to t5 of FIG. 3.
According to the results of analysis, it can be understood that the occurrence of the reverse current and the short-circuit problems caused by the shutdown of the forward converter are attributed to the delayed turn-off of the freewheel switch Q3. Hence, if it possible to timely turn off the freewheel switch Q3 when the input power is interrupted or before the resonance between the output inductor Lo and the output capacitor Co starts, the reverse current can be inhibited and the damages caused by the voltage spikes can be suppressed. To this end, it would be an ideal solution to devise a controller with a simple circuit architecture and cost-effectiveness to detect the shutdown of the forward converter or the occurrence of the reverse current and drive the freewheel switch Q3 to turn off in good time. The present invention can satisfy these needs.